Purpose of the study: Several studies have demonstrated the usefulness of mesenchymatous stem cells (MSC) for cell therapy aimed at favoring bone tissue healing. Bone morphogenesis proteins (BMP) orient MSC towards osteoblastic differentiation. Since they are rapidly degraded in the organism, these proteins require a continuous release system to potentialize their biological activity in a controlled localized manner. We evaluated the usefulness of using the electroporation technique to insert a BMP transgene into the MSC of rats to enable sufficient transient expression of BMP genes to enable satisfactory bone healing. We first developed electroporation conditions for rat MSC and checked cell viability after the electric shock. Secondly, in order to obtain quantitative and/or temporal BMP expression, we tested the influence of different promoters on transcription actvity.

Material and methods: To determine the electroporation parameters, MCS were transfected with the pCMV-LacZ plasmid using two electric impulsions: a series of eight 100 impulsions/μs at high voltage (900-170V/cm) followed or not by a series of eight 12.5 ms low-voltage impulsions (60 V/cm). After determining the electroporation conditions, six plasmids carrying different promoters were electroporated.

Results: The best transfection rate in rat MSC was obtained with a series of 8 impulsions at 1500 V/cm. Before the electrical shock, the suspended rat MSC had to be incubated at ambient temperature to favor cell survival. Proliferation of electroporated cells was comparable to that of non electroporated cells. Surprisingly, addition of low-voltage pulses significantly decreased the efficacy of transfection. In addition, MSC transfected with the promoters GAPDH and beta-actin presented a beta-galactoside activity (at 48 h) superior to that obtained with the pCMV promoter.

Discussion: After optimization of these parameters, we demonstrated that MCS can be effectively transfected by electroporation. The following steps will be to check for long-term expression of beta-galactoside by electroporated MSC, transfection of MSC with plasmids or the BMP-2 gene controlled by these same promoters and monitoring promoter activity as a function of the stage of MSC differentiation.